This invention relates to hydraulic steering systems, and, in particular, to a pressure compensated, non-feedback hydraulic steering system and valve.
Steering systems for heavy mobile equipment, particularly articulated vehicles that are used in construction, mining and logging, rely on hydraulic power controls for their operation. These power steering systems typically include a source of pressurized fluid, such as a pump, for delivering fluid under pressure to a pair of steering cylinders. The steering cylinders are connected to the wheels of the vehicle and are operable to steer the wheels. A directional control valve, usually a spool-type valve, controls the passage of fluid to and from the steering cylinders. Such spool valves usually have a neutral, centered position. They are shiftable to the left or right for operating the steering cylinders.
Large fluid forces act upon the directional control valve, so that some fluid power means is usually employed to operate the directional control valve. A typical steering system includes a steering wheel coupled to a hydraulic pump that operates a fluid actuator connected to the directional control valve. The steering wheel pump may be either manually driven or power assisted pump. Turning the steering wheel in a particular direction causes the fluid actuator to shift the directional control valve spool and thereby supply fluid to the head end of one steering cylinder and the rod end of the other cylinder.
One problem associated with hydraulic steering systems lies in giving the operator the control that is comparable to conventional steering wheel manipulations associated with a mechanical steering gear. In the latter, the turning radius remains constant when the operator stops rotating the steering wheel and the turn is completed by simply restoring the steering wheel to its initial position. In contrast, when the steering wheel of some hydraulic steering systems is stopped, the steering cylinders will continue to extend and contract causing the turning radius of the vehicle to continually decrease. The latter can only be stopped by returning the steering wheel to its initial position. This closes the directional control valve but leaves the steering cylinders at their respective extended and contracted positions so that the vehicle will make a constant radius turn. In order to end the turn, the steering wheel must be rotated in the opposite direction in order to reverse the operation of the steering cylinders and return the wheels to their initial position.
The foregoing mode of control, known as jerk steering, is difficult and imprecise. Accordingly, others have modified such a system to include a cancelling means that generates a feedback signal for cancelling the steering wheel input and simulating conventional steering. The feedback signal is transmitted from the articulated portion of the vehicle to the steering valve actuator by means of a linkage.
Such feedback linkages may be mechanical, hydraulic, or combined hydro-mechanical. An example of a variable feedback hydraulic linkage is shown in U.S. Pat. No. 4,003,202. There is shown a linkage including a pair of feedback cylinders which operate on the directional control valve actuator in order to cancel the steering wheel input in response to hydraulic signals derived from the steering cylinders.
The feedback linkage between the vehicle and the directional control valve is important because it cancels the steering wheel input and gives the operator a sense of steering similar to that of an automobile. Feedback linkages may come in various types and are applied in various ways, depending upon the kind of vehicle, the location of the operator, the steering valve and other parameters. Heretofore, feedback linkage components have been expensive and have required a high degree of precision in linkage location and attachment points or pivots. Although available feedback linkages provide good steering, all such linkages include some mechanism which eventually wears with age and thereby detracts from the high degree of precision that is usually demanded of such steering systems. Hence, it would be desirable to have a hydraulic steering system that steers in a conventional manner without a feedback linkage.